linux-kernel - Re: [RFC PATCH 0/2] tracing: Teach FETCH_MTD

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Date:	Wed, 27 Nov 2013 18:41:17 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH 0/2] tracing: Teach FETCH_MTD_symbol to handle
	per-cpu data

On 11/27, Masami Hiramatsu wrote:
>
> (2013/11/27 2:43), Oleg Nesterov wrote:
> >
> > This doesn't allow to read the data from other CPUs, but at least
> > the changes are simple and this_cpu_ is better than the reading
> > from the obviously wrong address.
>
> Yeah, usually per_cpu variable is used in current cpu context.
>
> >> For the dynamic allocated per-cpu things, it is also good to give
> >> a straight operation, like +10(percpu(%rdi)).
> >
> > Probably yes, but this needs more changes and I am still not sure
> > this is really useful. And if we do this, we probably also need
> > to allow to read from other CPUs...
>
> No, it is enough to provide "percpu(FETCHARG)" which just returns
> current cpu's percpu variable address.

I don't really agree. I am not saying this is terribly useful, but:

> (Note that kprobes handler
> runs in interrupt)

but this doesn't matter at all, I think. The code can execute, say,
__percpu_counter_sum-like code.

And even if we dump the .data..percpu memory (@percpu_symbol) the
user might want to know the "global" state of this per-cpu object.

And note that sometimes DEFINE_PER_CPU doesn't really connect to
smp_processor_id(). Just for example, bp_cpuinfo[]. It is never
used as this-cpu-data. This means that @bp_cpuinfo+whatever is
always pointless.

But anyway I agree, this is not that important, lets ignore.

Oleg.

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